In Orthogonal Frequency Division Modulation (OFDM) data is transmitted on multiple frequencies for the duration of a symbol time, T. A property of OFDM is that the individual carriers are spaced in frequency by 1/T, where T is the time duration of the data symbol. This property allows OFDM receivers to be made with ideal frequency selectivity between the individual carriers. In other words, the carriers in an OFDM waveform are spaced in such a manner that they effectively do not see each other at the receiver side, i.e. they are orthogonal to each other so that there is no cross-interference and hence no signal loss.
The benefits of OFDM are high spectral efficiency (throughput/MHz of channel bandwidth) and high resistance to multi-path interference and frequency-selective fading.
In order to retain the carrier orthogonality through multipath transmission channels with a delay spread, the OFDM symbols are effectively transmitted for a little longer time than T. At the receiver side this excess time is not needed for detecting the OFDM symbol and can therefore be regarded as a Guard Period (GP). For a typical system T can be 67 μs (Δf=15 kHz) and the GP can be 0.5 μs.
It is often convenient to define an OFDM system with carrier symmetry around the centre frequency of the channel, i.e. relative to the channel centre there are sub-carriers carrying data symbols at +/−N/2×Δf, where N is the total number of sub-carriers and Δf is the sub-carrier spacing. In this respect the 0 Hz carrier, or the DC carrier, is an exception because transmitter and receiver technology makes it difficult to avoid impairment on this carrier. For this reason the DC carrier is often sacrificed. From a spectral point of view, the DC carrier can be regarded as any other sub-carrier. The complex DC level, if constant in the duration of the symbol time, will not interfere with the other data symbols, i.e. the DC sub-carrier is orthogonal to the data carrying sub-carriers.
When designing an RF receiver for an OFDM system one choice may be a direct conversion receiver. In a direct conversion receiver, the RF signal is mixed directly to base band, i.e. the local oscillator frequency equals the channel frequency. The DC component, inherently generated in the RF mixers, will therefore be present at the DC sub-carrier and if the DC level is constant during the time duration of the symbol it will in principle be harmless to the detection of the data symbols in the other sub-carriers.
However, when small RF signals are received, the DC component relative to the wanted signal may be significant. In extreme cases the DC level might exceed the wanted signal level by an order of magnitude. As the dynamic range of the electronics, and in particular of the A/D converter, is limited it is therefore normally desirable to remove the DC component in the base band, before A/D conversion.
Unfortunately removing the DC component is not easily done without distorting the signal in such a way that it impacts the detection of the other sub-carrier symbols. This is because, for a number of reasons, the DC level typically changes during the time duration of a symbol. Changes in the DC level may be caused, for example, by strong external blockers in the receive band generating a signal in the DC sub-carrier. Also, the DC level typically changes when the gain is changed in the front-end and/or back-end of the receiver under the control of an Automatic Gain Control (AGC) system.
One method that can be used to remove the DC component is a narrow stop band filter (or high pass filter) centred at 0 Hz. The cut-off frequency of this filter may typically range from a small fraction of the sub-carrier spacing up to the sub-carrier spacing.
Using a High Pass Filter (HPF) with a very narrow cut-off frequency has the drawback that this filter has an impulse response much longer than the symbol time T. It will therefore introduce Inter Symbol Interference (ISI) that will degrade symbol detection. Also, the purpose of the filter, to the remove the DC, is not optimal as the settling time of the filter will be long and it therefore takes the duration of multiple OFDM symbols before the DC is reduced.
Using a HPF with a higher cut-off frequency, for example equal to n times the sub-carrier spacing has the drawback that it breaks the orthogonality of the sub-carriers to the DC carrier. The equalization based on the transmission channel estimate, as commonly used in the digital receiver, will not be able to remove the distortion caused by the HPF. Another view on this problem is that the DC level after the HPF will not be constant throughout the OFDM symbol time because of the HPF settling. When the DC is not constant, the power spectrum will spread to the other sub-carriers and distort the modulated symbols in these sub-carriers.
It is an aim of some embodiments of the present invention to address, or at least mitigate, some of these problems.